1. Field of the Invention
The present invention pertains to process management, and more particularly, to a system and method for monitoring and adjusting one or more process inputs, including determining and utilizing an optimum set point of one or more selected variables, to achieve optimization of one or more outputs.
2. Description of the Related Art
The purpose of process control is to optimize inputs and outputs. A representative activity that invokes these goals is the processing of raw or semi-finished materials for use in the manufacture of goods. In this context a manufacturing process is defined as inputs (materials, components, and manual or machine operations, automatic or manual) that are combined to create a product. Although the disclosed embodiments of the invention are discussed and described in this context, it is to be understood that the present invention will have application in other processes having the same or similar objectives.
All manufacturing processes exhibit variation of inputs, resulting in end product output variation. Output variation needs to be within certain parameters to create a product that meets an end user's specifications. Variation in inputs can take the form of naturally occurring variation about a central mean, or it can be variation that is controlled or changed by varying inputs. Such inputs can be quantity of material, line speed, temperature, moisture, line speed/cycle time, thickness, density, catalysts, accelerants, adhesives, resins, or other inputs, such as sharpening a tool, moving a press roll, calendar roll, adjusting an extruder die, etc.
If it were not for variation, inputs could be combined to yield a perfect or predictable output (product), such as the production of flat glass, from silica, on a float glass line. Because of input variation the glass output or end product will have characteristics that vary, e.g., thickness, strength, weight, and optical quality.
Providers of raw materials and the manufacturers of goods are faced with the challenge of manufacturing products that meet their customers' strict quality requirements while attempting to minimize production costs and optimize outputs. The product characteristics, such as thickness or strength, generally must fall within a specified range of quality measurements. Failure to meet quality requirements will require scrapping of the materials or additional labor and materials to rework the product. Accordingly, a manufacturer must then manage the process in order to produce a product having a target point that falls within the specified range. Normal process management does not intentionally produce defective product, and it accepts variation in output resulting from variation in input.
A product's target point is the desired point of the product's quality within the specified range of quality measurement. Where the target point falls within the specified range has a significant impact on production costs. The higher the production costs, the lower the financial margins realized in the production of a given product or set of products.
Generally any output falling within the range of specifications is acceptable. Output outside of this range is not acceptable. It is normal manufacturing practice to set or to select inputs such that all outputs fall within the acceptable range. It is also common practice to set an input target at a value and allowable variation that is economically justified, based on the process capability of the process producing that input.
As an example, seven-ply veneer plywood, with a specified thickness of 0.750 inches, plus or minus 0.030 inches can be easily produced within specification, if each of the seven plies of veneer can be produced to a target thickness of plus or minus 0.001 inches. Unfortunately the acceptable tolerance for green veneer production is plus/minus 0.005 inches or greater. The seven plies of green veneer alone yield a plywood panel thickness variation of plus/minus 0.035 inches, exceeding the product specification of plus/minus 0.030 inches. The thickness variation in subsequent processing of veneer drying, resin application and hot pressing is in addition to the green veneer variation, further exceeding the product specification.
A large percentage of products in the foregoing example fall outside (below) the customer lower specification limit (LSL) or above the upper specification limit (USL), as illustrated in FIG. 1. A subsequent process operation of finish panel sanding will reduce panel thickness, bringing thicker panels that are above the USL, within specification. This operation will also reduce the thickness of some of the panels that are within the specification limit, to go below the LSL. One solution to this problem is to peel the veneer thicker so that the target thickness of the veneer is almost always above the LSL. If the laid up target thickness of the veneer is selected to be 3 standard deviations above the LSL for the panel, only a small percentage of the pressed plywood panels will be below the LSL. In this solution however, a much higher percentage of pressed plywood panels will be above the USL. At three sigma, approximately 99.85% of the finished plywood panels will be above the LSL. This does mean, however, that 1,350 panels out of each million produced will still be below the LSL. Setting sigma to six means that only one out of every billion panels will be below the LSL. This solution will require a great deal of extra veneer and additional finished panel sanding. For panels with a thin facing of expensive veneer (hardwoods), thicker panels will have some of the outside face entirely sanded away, exposing the core. These panels must be scrapped or downgraded. This solution requires extra veneer, energy for drying the extra veneer, energy for pressing, additional press cycle time, sanding expense, and has a negative environmental impact.
Other processes are similar in concept but different in application. Common to these processes, is setting the process inputs, such that the resulting output is within the LSL and USL. These solutions require excess costs, produce a product with more variation, and sacrifice in output quality. In processes where there is only a minimum, or LSL, the process is adjusted to be above this LSL, to ensure that the customer is given fair measure. Examples include food, tire or panel strength, paper weight and strength, ceiling tile thickness and weight, and gypsum wallboard thickness and weight.